1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
FIG. 1 illustrates a fixing device 110 installed in such image forming apparatuses, which includes a fixing roller 101 and a pressing roller 102 that apply heat and pressure to a recording medium P bearing a toner image. For example, the pressing roller 102 is pressed against the fixing roller 101 heated by a heater 106 disposed inside the fixing roller 101 to form a fixing nip N therebetween through which the recording medium P bearing the toner image is conveyed. As the fixing roller 101 and the pressing roller 102 rotate and convey the recording medium P through the fixing nip N in a recording medium conveyance direction D, the fixing roller 101 and the pressing roller 102 apply heat and pressure to the recording medium P, melting and fixing the toner image on the recording medium P.
A thermistor 112 contacts the outer circumferential surface of the fixing roller 101 to detect the temperature of the fixing roller 101 so that a controller controls the heater 106 to heat the fixing roller 101 to a desired temperature based on the temperature detected by the thermistor 112. Additionally, a plurality of separation pawls 111 contacts the outer circumferential surface of the fixing roller 101 to separate the recording medium P discharged from the fixing nip N from the fixing roller 101. FIG. 2 illustrates the arrangement of the thermistor 112 and the separation pawls 111. As shown in FIG. 2, the four separation pawls 111 and the single thermistor 112 are aligned in the axial direction of the fixing roller 101.
As the recording medium P bearing the toner image is conveyed through the fixing nip N, toner of the toner image may adhere from the recording medium P to the fixing roller 101 due to heat from the fixing roller 101. As the separation pawls 111 and the thermistor 112 slide over the rotating fixing roller 101, they scratch toner adhered to the fixing roller 101. Conversely, bands on the outer circumferential surface of the fixing roller 101 where the separation pawls 111 and the thermistor 112 do not slide over the fixing roller 101 continue carrying toner adhered from the toner image on the recording medium P, producing toner streaks on the outer circumferential surface of the fixing roller 101 as shown in FIG. 2. Accordingly, as the subsequent recording medium P is conveyed through the fixing nip N, the toner streaks produced on the fixing roller 101 are transferred onto the subsequent recording medium P, producing glossy streaks 113 on the subsequent recording medium P as shown in FIG. 2. Further, toner caught and accumulated by the separation pawls 111 and the thermistor 112 may fall onto the fixing roller 101 and may be further transferred from the fixing roller 101 onto the recording medium P, thus staining the recording medium P.
FIG. 3 is a graph illustrating a relation between the number of prints performed by the fixing device 110 shown in FIG. 1 and a level of glossy streaks that appear on recording media P. As shown in FIG. 3, the greater the glossy streak level, the smaller the amount of glossy streaks that appear on the recording media P. By contrast, the smaller the glossy streak level, the greater the amount of glossy streaks that appear on the recording media P. The dotted line defines the acceptable limit of the gloss streak level of 3.5. As the number of prints increases, the glossy streak level deteriorates. Specifically, with the number of prints of 2,000 sheets or more, the glossy streak level is below the acceptable limit of 3.5.
To address the above-described drawbacks of the fixing device 110, a fixing device that removes toner adhered from the recording medium P to the fixing roller 101 is proposed. For example, the fixing device includes a sliding roller that slides over the outer circumferential surface of a fixing roller to pick up toner therefrom and a cleaning roller that picks up toner from the sliding roller. However, the sliding roller sliding over the fixing roller may damage the outer circumferential surface of the fixing roller.